1. Field of the Invention
This invention relates generally to the controlled cooling of hot rolled steel products such as rods and the like in direct sequence with the rolling operation in order to achieve predetermined metallurgical properties, and is concerned in particular with an improved apparatus and method for increasing the rate at which such products may be air cooled.
2. Description of the Prior Art
The controlled air cooling of hot rolled steel rod in direct sequence with the rolling thereof began approximately twenty years ago with the process described in U.S. Pat. No. 3,231,432 (McLean et al). This process involves hot-rolling the rod and thereafter directly coiling it onto an open conveyor in spread out ring form while the microstructure of the steel is still in a condition of highly uniform, relatively small austenite grain size. While moving along the conveyor, the rings are air cooled through allotropic transformation. This produces a microstructure sufficiently equivalent to that achieved by air or lead patenting so as to enable the rod to be subsequently processed to a finished product, as for example by being drawn into wire, without additional heat treatment.
In the earlier installations of this process, chaintype conveyors were employed. However, because of the tendency of the rings to undergo scratching as a result of their being dragged over stationary support rails located between the chains, and because prolonged area contact with such rails produces non-uniform cooling, the use of chain-type conveyors was eventually discontinued to a large extent in favor of roller conveyors of the type shown for example in U.S. Pat. No. 3,930,900 (Wilson). Here, the rings are transported over driven rollers, with air nozzles arranged between the rollers to blow cooling air upwardly through the rings. The rod sizes that are processed on installations of this type typically range from about 5-19 mm. in diameter, and the typical cooling rates that can be achieved at static water pressures of between about 7"-10" are shown by the curves x.sub.1 and x.sub.2 of FIG. 1. Curve x.sub.1 represents the cooling rate at the sides of the conveyor where the overlapped rings are more densely packed, as compared to the center of the conveyor, where the ring density is less and the cooling rate is more rapid, as represented by curve x.sub.2. The vertical distance between curves x.sub.1, x.sub.2 is an indication of the non-uniformity of cooling being experienced for a particular rod size. It will be observed that as the rod size increases, there is a decrease in the cooling rates. This is due to the decrease in the ratio of surface area to volume which characterizes the larger rod sizes. For high carbon steels such as for example AISI 1085, the cooling rates of curves x.sub.1, x.sub.2 yield average tensile strengths depicted by the curve "X" of FIG. 2. When compared with curve "Z" of FIG. 2, which depicts the tensile strengths achievable with conventional lead patenting, the results depicted by curve X are uniformly lower for all rod sizes.
Improvements in uniformity of cooling and flexibility of operation have been achieved by arranging the air cooling nozzles directly under the conveyor rollers, as shown for example in U.S. Pat. No. 4,448,401 (Jalil et al). However, unless static air pressures are increased significantly, which of course increases power consumption and operating costs, such arrangements do not increase the rate at which the rings are cooled.
Attempts also have been made at achieving increased cooling rates by employing water as a cooling medium. See for example U.S. Pat. No. 4,395,022 (Paulus et al) which describes an apparatus for cooling hot rolled steel products, including rod, by immersion in a water bath. Cooling by water immersion has reportedly achieved somewhat accelerated cooling rates with improved tensiles for larger rod sizes. However, uniform results have been difficult to achieve. This is due to the difficulty of maintaining optimum water chemistry, a problem which is compounded by the need to continuously remove contaminants such as dirt, mill scale, etc. from the water bath. Experiments also have been conducted with water sprays, but here again uniformity has proven to be elusive.
Thus, the cooling curves x.sub.1, x.sub.2 of FIG. 1 and the resulting average tensile strengths X of FIG. 2 remain representative of current commercial practice when rapidly cooling hot rolled steel rod in direct sequence with the rolling operation.
This has necessitated certain compromises on the part of rod producers. More particularly, when producing rod sizes below about 9 mm., the average tensile strengths of curve X have been considered as being acceptable for most commercial purposes, despite the fact that they are significantly lower than those attainable by off-line processes such as lead patenting (curve Z). However, when producing rod sizes of 9 mm. and above, the tensile strengths of curve X are considered to be unacceptable. Consequently, most mills either draw wire to greater reductions, or use alloying elements to increase the hardenability of the steel, or resort to off-line lead or salt patenting heat treatments. The first of these alternatives yields mixed results, and the second and third alternatives significantly increase tonnage costs.
It thus will be seen that, the prior art has failed to satisfactorily meet the demands of the industry when processing the larger rod sizes ranging from 9 to 19 mm. in diameter.